The major point is that the implications of present and prospective population growth remain essentially the same, whichever of the projections is considered. In the long run such rates of increase cannot possibly persist because the limit of population growth is set by the finite dimensions of the planet. A special caution on the data for China is necessary. These data are especially defective by reason of the absence of good census statistics and reliable vital statistics. The estimated population for China by 2000, according to different United Nations assumptions, could vary from less than 900 million to about 1.4 billion. Other estimates indicate even greater variation is possible. Obviously the data in Table 2 for east Asia and for the world as a whole are vitally affected by the weakness of the data for China. AGE STRUCTURE AND DEPENDENCY Overall numbers and growth rates do not explain all the important changes in the demographic transition. As birth and death rates change, so do the age structure and dependency ratios—the proportion of dependents who are under 15 or over 60. The expectation of life for newborn infants and for those who survive to age 20 also changes. Table 3 presents a model of demographic profiles of four populations— from the premodern period of high birth and death rates, through the transitional stages in which death rates decrease, to the modern phase of low birth and death rates. In the later transitional period-through which the fast-growing regions of the world are now passing—the death rate declines and the birth rate remains at a relatively high level. Because of better health conditions and increased survival of women of childbearing age, infants, and children, the proportion of young people under 15 rises to about 45 percent of the population. At the same time, of course, the proportion of old people and of working-age people declines. As the demographic transition proceeds and birth rates begin to decline along with continuing declines in mortality, the demographic profile becomes that of a modern population. Growth rates, age structure, life expectancy, and dependency ratios change radically, as shown in the last two columns of Table 3. The relevance of these population models is evident when we examine actual populations. In 1960 about two thirds of the less developed countries had birth rates ranging from 40 to 50. Expectation of life at birth among most of these countries ranged from 30 to 60 years. Population growth rates ranged from 1 to 3.5 percent per year. In 1960, 40 percent of the population was under 15 years of age; only 3.3 percent of the population was over 65; and 56.6 percent was 15 to 64 years old. Hence, in 1960 in the less developedat the population in the developed areas would have shrunk to 23 percent.